Ventilation is the process of air exchange, removing inside air from inside the plant production facility and replacing it with outside air. Ventilation is also important for maintaining CO2 levels and removing excess moisture when there are no active systems installed to manage those factors. Ventilation is commonly used as the first stage of cooling in greenhouses. It can also be used for vertical farms to limit energy used by compressors when the outdoor conditions are favorable.
The total volume of air that moves through the greenhouse (or indoor facility) in one hour is referred to as the Air Change Rate per Hour (ACH). The ACH required depends on several factors, including outdoor climate and the use of heating/cooling systems. In general, ventilation rates should be optimized to balance carbon dioxide enrichment during heating, to remove excess moisture when the it's cold outside, and to eliminate excess heat during cooling. Crop layout and row orientation will affect the direction and uniformity of airflow.